Semiconductor device manufacturing: process – Making device or circuit responsive to nonelectrical signal
Reexamination Certificate
2001-01-05
2002-10-15
Mulpuri, Savitri (Department: 2812)
Semiconductor device manufacturing: process
Making device or circuit responsive to nonelectrical signal
C438S053000, C361S283400, C073S724000
Reexamination Certificate
active
06465271
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The invention relates to pressure sensors and, more particularly, touch mode capacitive pressure sensors.
BACKGROUND OF THE INVENTION
In various industrial and commercial applications, it is desired to measure pressure in a hostile environment, with a miniaturized sensor having good stability, low power consumption, robust structure, large over pressure protection range, and good linearity and sensitivity. For example, such a sensor could be used in conjunction with an RF transponder disposed within a pneumatic tire as shown in commonly-owned, copending PCT patent application No. US98/07338 filed Apr. 14, 1998, incorporated in its entirety by reference herein. Applications such as the pneumatic tire place additional requirements on the pressure sensor due to the need for the sensor to withstand both the normal operating temperature and pressure ranges and also the much higher (many times the operating values) manufacturing temperature and pressure. For example, molding the sensor into a tire is but one illustration of an environment where conventional sensors fail to meet these desired criteria.
Capacitive pressure sensors are known, and can be designed to meet many if not all of the desired characteristics. Capacitive pressure sensors generally include two capacitive elements (plates, or electrodes), one of which is typically a thin diaphragm, and a gap between the electrodes. When a pressure is exerted on the diaphragm, the diaphragm deflects (deforms) and the size of the gap (in other words, the distance between the two capacitive elements) varies. And, as the gap varies, the capacitance of the sensor varies. Such changes in capacitance can be manifested, by associated electronic circuitry, as an electronic signal having a characteristic, such as voltage or frequency, indicative of the pressure exerted upon the sensor.
In the “normal” operation mode of a capacitive pressure sensor, the diaphragm does not contact the fixed electrode. The output capacitance is nonlinear due to an inverse relationship between the capacitance and the gap which is a function of pressure P. This nonlinearity becomes significant for large deflections. Many efforts have been made to reduce the nonlinear characteristics of capacitive sensors either by modifying the structure of the sensors or by using special non-linear converter circuits.
Particularly advantageous for the achievement of linearity has been the development of “touch mode capacitive pressure sensors” (TMCPS). A particular class of capacitive pressure sensors operate in what is known as “touch mode”. Touch mode sensors have been disclosed, for example, in Ding, et al.,
Touch Mode Silicon Capacitive Pressure Sensors
, 1990 ASME Winter Annual Meeting, Nov. 25, 1990, incorporated in its entirety by reference herein. They are further explained in Ko and Wang,
Touch Mode Capacitive Pressure Sensors
, Sensors and Actuators 2303 (1999), also incorporated in its entirety by reference herein.
Conventional capacitive pressure sensors normally operate in a pressure range where the diaphragm is kept from contacting the underlying electrode, and normally exhibit nonlinear characteristics. This inherent non-linearity has led to the development of many linearization schemes using complex and costly interface circuits which include analog circuits and amplifiers, segment linearization, microprocessor and ROM matrix linearization, etc.
In contrast thereto, touch mode capacitive pressure sensors, operating in the range where the diaphragm touches the insulating layer on the underlying electrode, exhibit near linear behavior in certain pressure ranges. The increased linearity is attributable to the touched area (footprint) increasing linearly with applied pressure and the increased rigidity of the diaphragm after touch.
Furthermore, the touch mode capacitive pressure sensor has much higher sensitivity (large capacitance change per unit pressure change) compared to conventional capacitive pressure sensors. Therefore, small environmentally-caused capacitance changes over time become insignificant and can be neglected. This makes the touch mode device a long term stable device over a wide range of environmental conditions.
These advantages are inherent with touch mode capacitive devices, no matter what materials are used for the diaphragm and the substrate.
Generally, touch mode capacitive pressure sensors differ from conventional capacitive pressure sensors (described hereinabove) in that the diaphragm element is permitted to deflect sufficiently to come into actual physical contact with the underlying fixed capacitive element at a given pressure. Typically, a thin dielectric insulating layer on the fixed capacitive element prevents the diaphragm from electrically shorting to the fixed capacitive element. As the pressure increases, the “footprint” of the diaphragm upon the fixed capacitive element increases, thereby altering the capacitance of the sensor, which can be manifested, by associated electronic circuitry, as an electronic signal having a characteristic indicative of the pressure exerted upon the sensor. The major component of the touch mode sensor capacitance is that of the touched area footprint where the effective gap is the thickness of the thin insulator layer between the pressed-together capacitive elements. Because of the small thickness and large dielectric constant of the isolation layer, the capacitance per unit area is much larger than that of the untouched area which still has an added air or vacuum gap. In a certain pressure range, the touched area is nearly proportional to the applied pressure, and results in the nearly linear C-P (capacitance-pressure) characteristics of the touch mode pressure sensor. For the range of pressures in the touch mode operation region, the sensor capacitance varies with pressure nearly linearly and the sensitivity (dC/dP) is much larger than that in the near linear region of a normal mode device. In addition to high sensitivity and good linearity, the fixed element substrate provides support to the diaphragm after it touches, thus enabling the device to have significant pressure over-load protection. In summary, the advantages of TMCPS are nearly linear C-P characteristics, large overload protection, high sensitivity and simple robust structure that can withstand industrial handling and harsh environments.
As used herein, a “touch mode” capacitive pressure sensor includes any capacitive pressure sensor wherein at least a portion of the operating range of the pressure sensor occurs while the diaphragm is in physical contact with the underlying capacitive element.
An early example of a capacitive touch mode pressure sensor is shown in U.S. Pat. No. 3,993,939, entitled PRESSURE VARIABLE CAPACITOR, incorporated in its entirety by reference herein. This patent discloses a large scale version of TMCPS with a variety of diaphragm constructions.
Some of the more recent efforts have focused on miniaturization, cost reduction, and performance improvements. An example is shown in U.S. Pat. No. 5,528,452, entitled CAPACITIVE ABSOLUTE PRESSURE SENSOR, incorporated in its entirety by reference herein. This patent discloses a sensor comprising a substrate having an electrode deposited thereon and a diaphragm assembly disposed on the substrate.
The TMCPS diaphragm can be made of different materials, such as silicon, poly-silicon, silicon nitride, polymeric materials, metal, and metallized ceramic. Each material choice has its advantages and disadvantages. For good stability, robust structure, and avoidance of temperature and pressure related problems, the use of single crystal silicon is preferred, because it has well characterized, well understood, reliable and reproducible electrical and mechanical properties. The aforementioned U.S. Pat. No. 5,528,452 discloses a preferred embodiment with a single crystal silicon diaphragm, which is electrostatically bonded (anodic bonding) to a glass substrate. Although this provides a simple construction which avoids prior art problems with seali
Ko Wen H.
Wang Qiang
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